The goal of this research plan is to translate the progress made with prior NIH funding into a completely biological tissue-engineered pulmonary valve replacement, made of tissue grown in vitro from fibroblast remodeled fibrin. This will be accomplished using (1) advances in heart valve bioreactor design and operation to strengthen the root segment and the root-leaflet attachment line, and (2) a new decellularization and optional recellularization strategy using mesenchymal stem cells to obviate the leaflet contraction that occurred post-implantation due to the transplanted fibroblasts, which are used to grow the tissue valve in vitro. This tissue-engineered heart valve (TEHV), both decellularized (relying on host cell invasion) and recellularized (with mesenchymal stem cells pre-implantation) will be validated in a lamb model to demonstrate growth capacity and sustained function of the engineered valve. Success will ultimately benefit approximately 10,000 pediatric patients in the U.S. annually and ultimately 100,000 patients in the U.S. annually if this TEHV can subsequently be improved to withstand forces associated with the aortic valve position. Enabling technologies relevant to other cardiovascular tissue engineering applications will be generated as a by-product of this research.